The detection of carbon dioxide (CO2) gas has attracted attention in the context of global warming, biological and health-related applications such as indoor air quality control, process control in fermentation, and in the measurement of CO2 concentrations in patients' exhaled breath with lung and stomach diseases.
In medical applications, it can be critical to monitor the CO2 and O2 concentrations in the circulatory systems for patients with lung diseases in the hospital. The current technology for CO2 measurement typically uses IR instruments, which can be very expensive and bulky.
The most common approach for CO2 detection is based on non-dispersive infrared (NDIR) sensors, which are the simplest of the spectroscopic sensors. The best detection limits for the NDIR sensors are currently in the range of 20-10,000 ppm. The key components of the NDIR approach are an infrared (IR) source, a light tube, an interference filter, and an infrared (IR) detector. In operation, gas enters the light tube. Radiation from the IR light source passes through the gas in the light tube to impinge on the IR detector. The interference filter is positioned in the optical path in front of the IR detector such that the IR detector receives the radiation of a wavelength that is strongly absorbed by the gas whose concentration is to be determined while filtering out the unwanted wavelengths. The IR detector produces an electrical signal that represents the intensity of the radiation impinging upon it. It is generally considered that the NDIR technology is limited by power consumption and size.
In recent years, monomers or polymers containing amino-groups, such as tetrakis(hydroxyethyl)ethylenediamine, tetraethylene-pentamine and polyethyleneimine (PEI) have been used for CO2 sensors to overcome the power consumption and size issues found in the NDIR approach. Most of the monomers or polymers are utilized as coatings of surface acoustic wave transducers. The polymers are capable of adsorbing CO2 and facilitating a carbamate reaction. PEI has also been used as a coating on carbon nanotubes for CO2 sensing by measuring the conductivity of nanotubes upon exposing to the CO2 gas. For example, CO2 adsorbed by a PET coated nanotube portion of a NTFET (nanotube field effect transistor) sensor lowers the total pH of the polymer layer and alters the charge transfer to the semiconducting nanotube channel, resulting in the change of NTFET electronic characteristics.
The current technology for O2 measurement, referred to as oximetry, is small and convenient to use. However, the O2 measurement technology does not provide a complete measure of respiratory sufficiency. A patient suffering from hypoventilation (poor gas exchange in the lungs) given 100% oxygen can have excellent blood oxygen levels while still suffering from respiratory acidosis due to excessive CO2. The O2 measurement is also not a complete measure of circulatory sufficiency. If there is insufficient blood flow or insufficient hemoglobin in the blood (anemia), tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive. The current oxide-based O2 sensors can operate at very high temperatures, such as the commercialized solid electrolyte ZrO2 (700° C.) or the semiconductor metal oxides such as TiO2, Nb2O5, SrTiO3, and CeO2 (>400° C.). However, it remains important to develop a low operation temperature and high sensitivity O2 sensor to build a small, portable and low cost O2 sensor system for biomedical applications.